In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful for the preparation of lithographic printing plates typically comprise an imageable layer over the hydrophilic surface of a substrate. The imageable layer comprises one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder.
If after exposure to radiation the exposed regions of the imageable layer are removed in the developing process revealing the underlying hydrophilic surface of the substrate, the element is positive working. Conversely, if the developing process removes the unexposed regions and the exposed regions remain, the element is negative working. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive and the regions of the substrate surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
Direct digital imaging of printing plate precursors, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. Negative working imageable elements useful as lithographic printing plate precursors that can be imaged with infrared lasers are described, for example, in EP-A-0 672 544; EP-A-0 672 954; DeBoer, U.S. Pat. No. 5,491,046; and EP-A-0 819 985. Some of these elements require a post-exposure bake, that is, after imaging and before developing, the imaged element must be heated briefly to a temperature of about 85 to about 135° C. to cure the imaged regions. This requires a large amount of energy, necessitates use of a large processor for development, and limits throughput.
Despite improvements that have been made in negative-working imageable elements there is a continuing need for elements that do not require a post-exposure bake, and for elements that have higher speed, that is elements that require less energy for imaging.